As a technology to detect zero-crossing points (zero-crossing timings) of an AC voltage, there has been known a technology disclosed in JP-A-2010-239774, for example. JP-A-2010-239774 discloses a technology to detect zero-crossing points using a photocoupler.
However, according to a method of detecting zero-crossing points using a photocoupler as in JP-A-2010-239774, it is possible to appropriately detect zero-crossing points, but the power consumption of a photodiode of the photocoupler is not negligible. For this reason, a technology to detect zero-crossing points which can save electric power has been desired.